Attention-deficit hyperactivity disorder (ADHD) is a childhood disorder characterized by inattention and/or hyperactivity-impulsivity. According to the diagnostic and statistical Manual of Mental Disorders, fourth edition (DSM-IV; American Psychiatric Association, 1994), it affects approximately 3-5% of all school-aged children. Currently, little is known about the etiology of ADHD or the sex difference in its prevalence. Past twin studies have shown that the magnitude of genetic influences on ADHD is moderate to high (e.g., Goodman 7 Stevenson, 1989). Several studies have examined possible sex differences in the etiology of ADHD or the sex difference in the prevalence of ADHD, but unfortunately, these studies report conflicting results. One factor that may be contributing to the variability in the results is that ADHD has two dimensions, inattention and hyperactivity-impulsivity, but most researchers have examined ADHD as one dimension. In order to address this issue, the two dimensions of ADHD will be examined in studying the etiology of ADHD, possible sex differences in the etiology of ADHD, and the etiology of the sex difference in the prevalence of ADHD, using confirmatory factor analysis and univariate and multivariate behavior genetic analyses.